Hollow fibers and hollow fiber membranes and their use in the technical or medical area have been known for a long time. Such hollow fibers are produced by known spinning processes, and must always be spun with an internal filler (lumen filler) to stabilize the lumen of the hollow fibers during spinning. This lumen filler can be a gaseous or a liquid substance.
Most commercial hollow fiber membranes are currently manufactured with liquid lumen fillers, especially isopropyl myristate (IPM) or similar compounds to stabilize the lumen.
Because of its insolubility in water and its nontoxic properties, isopropyl myristate is outstandingly suitable as a lumen filler or lumen stabilizer, especially in the manufacture of hollow cellulose fibers which, as is known, are produced in aqueous systems. Isopropyl myristate is also characterized by an advantageous ease of handling. Like all other liquid lumen fillers, the lumen filler must be removed prior to later use of such hollow fibers in the medical and for the most part in the technical area as well. In hollow fibers, especially those used in the medical area (hemodialysis, hemofiltration, sterile filtration, plasmapheresis, etc.) this must be done prior to the pobting procedure of the hollow fibers. Special attention must be given to the fact that the usual pore stabilizer glycerine or other polyalcohols, added in many cases, must not be removed from the membrane. Likewise, other auxiliary agents such as hydrophilizing agents (for example polyvinylpyrrolidone, polyethylene glycol) must not be removed from the membrane, since they contribute to stabilization of the membrane. However, undesired auxiliary processing agents (thickeners, precipitating agents, etc.) must be removed from the finished membrane, since the membrane otherwise cannot be used for the applications for which it is intended.
In addition, the membrane polymer must not be altered chemically and in particular the structure of the membrane must be retained.
Thus far, chlorofluorocarbons such as Freon (R113) or Kaltron have been used to remove internal fillings from hollow fibers (DE-A-28 18 214). These substances result in a washout of the lumen filler used previously, such as isopropyl myristate. However, within the framework of environmental problems (damage to the ozone layer, etc.), it must be assumed that chlorofluorocarbons will no longer be permitted to be used in the foreseeable future. Hence, there is a demand for a substitute which, like chlorofluorocarbons is likewise able to remove the lumen filler from the hollow fibers to such an extent that the residual amount of substances remaining to be removed does not have a negative effect on further processing of the membrane and subsequent use.
Hexane, cyclohexane, N-octane, and various glycerine-alcohol mixtures have been used as alternatives to the chlorofluorocarbons. Apart from the fact that these substances to some extent do not have as powerful a cleaning action as the chlorofluorocarbons, they also have the disadvantage of flammability due to the formation of explosive mixtures with air.
According to DD-A-289 206 and DD-A-289 207, hollow fibers can also be cleaned with water provided the lumen filler used is one that is displaceable by water and the membrane polymer is inert with respect to water.
Flushing hollow fibers with a gas dissolved under pressure in a liquid is described in NL-A-76 04 567 while flushing with gases is discussed in U.S. Pat. No. 4,540,490, U.S. Pat. No. 4,935,143, JP 54067574, and PCT/WO 85 01449. However this process suffers from the disadvantage that it is limited to special lumen fillers and types of hollow fiber.